The ethylene-based ionomer is a resin obtained by intermolecular bonding between molecules of a copolymer of ethylene and an unsaturated carboxylic acid such as (meth)acrylic acid with a metal ion such as sodium or zinc and has long been known as a resin having excellent heat sealability, transparency, toughness, abrasion resistance, and low temperature resistance (PTL 1), and for example, it is used as a material for golf balls.
At present, as commercially available ethylene-based ionomers, “SURLYN (registered trademark)” developed by DuPont, and “HIMILAN (registered trademark)” sold by DuPont-Mitsui Polychemical Co., Ltd. in Japan are known. As the ethylene/(meth)acrylic acid copolymer used in the ethylene-based ionomer known in the related art, specifically, a polar group-containing olefin copolymer obtained by polymerizing ethylene and a polar group-containing monomer such as (meth)acrylic acid by a high pressure radical polymerization method is used.
On the other hand, in a polymerization method using a catalyst such as a general metallocene catalyst, which is typically known as another production method of a polar group-containing olefin copolymer, it is known that if acrylic acid, methacrylic acid, or the like is used as the polar group-containing monomer, these polar group-containing monomers become catalyst poison, and thus, polymerization activity is extremely lowered, and due to this, a desired polar group-containing olefin copolymer cannot be obtained.
Therefore, as a modification of resins using an ethylene-based ionomer, a method of blending a high density polyethylene resin in an ethylene-based ionomer by a high pressure radical polymerization method in the related art (PTL 2), a method of copolymerizing other comonomer components (PTL 3), and the like have also been proposed.
In addition, examination in which using a chromium catalyst, a copolymer of ethylene and an acrylic acid ester is produced in the presence of a Lewis acid, then, the obtained polar group-containing olefin copolymer is hydrolyzed with sodium hydroxide to modify into an ethylene/acrylic acid copolymer, and by reacting with metal ions, an ethylene-based ionomer is produced has also been reported (PTL 4).